The present invention relates to polymeric films. More specifically, the present invention relates to hydrophilic films useful for ink jet printing.
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages including melt film formation, quenching and windup. For a general description of these and other processes associated with film-making, see K R Osborn and W A Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
An optional part of the film-making process is a procedure known as xe2x80x9corientation.xe2x80x9d The xe2x80x9corientationxe2x80x9d of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of xe2x80x9corientationxe2x80x9d is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength. Biaxial orientation is employed to more evenly distribute the strength qualities of the film in two directions. Biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces and tearing than non-oriented films or films oriented in only one direction.
Ink jet printers, that is to say printers which form an image by firing a plurality of discrete drops of ink from one or more nozzles onto the surface of a recording sheet placed adjacent the nozzles, have recently enjoyed a large increase in sales. Such ink jet printers have the advantage that they can reproduce good quality text and images, in both monochrome and full color, can produce both reflection prints and transparencies, and are relatively inexpensive to manufacture and to operate, as compared with, for example, color laser printers, thermal wax transfer printers and dye sublimation printers. Accordingly, ink jet printers now dominate the home/small office market, and are often also used to provide color capability not available from the monochrome laser printers typically employed in larger offices.
Although, modern ink jet printers can print on almost any conventional paper or similar medium, and indeed are routinely used with commercial photocopying paper for printing text, the quality of images produced by such printers is greatly affected by the properties of the medium used. To produce high quality images reliably, it is necessary that the medium (ink jet recording sheet) used rapidly absorbs the ink, in order that the ink does not remain wet for an extended period, since otherwise the ink is likely to smear when successive sheets are stacked in the output tray of the printer. On the other hand, the medium should not promote excessive spreading of the ink droplet, since such spreading reduces image resolution and may result in color distortion if adjacent ink droplets intermix. The medium must be capable of absorbing the ink without substantial distortion of the medium, since otherwise unsightly xe2x80x9ccocklingxe2x80x9d (formation of ripples and similar folds) may occur, and most observers find such distortions unacceptable. Once the ink has dried, the medium should be such that contact of the image with moist surfaces (such as sweaty fingers) does not result in bleeding of ink from the image. Finally, since the surface characteristics, such as smoothness, glossiness and feel, of the image are largely determined by the same characteristics of the medium, the medium should possess characteristics appropriate to the type of image being printed. When, as is increasingly common, an ink jet printer is used to print a digital image produced by a camera or a scanner, the medium should be smooth and possess the high gloss and smooth feel of conventional silver-halide based photographic printing paper.
It is difficult to reconcile all these demands upon an ink jet printing medium and, as shown by the literature, much research has been dedicated to improving such media. For example, U.S. Pat. No. 4,592,951 describes an ink jet recording sheet comprising a transparent support carrying a layer of cross-linked poly(vinyl alcohol).
U.S. Pat. No. 4,904,519 describes an ink jet recording sheet comprising a transparent polymeric backing having on at least one major surface thereof a transparent, ink-receptive layer comprising a cross-linked, hydrolyzed copolymer of a vinyl ester comonomer selected from the group consisting of vinyl acetate, vinyl propionate and vinyl stearate, and a vinyl amide comonomer selected from the group consisting of N-vinyl pyrrolidone and vinyl acetamide, the degree of hydrolysis being from about 80-95%, and the cross-linking being effected by an agent selected from the group consisting of borates, titanates, dichromates and aldehydes.
U.S. Pat. No. 4,900,620 describes an ink jet recording sheet including a sheet-like substrate composed mainly of 70 to 100 wt % of wood pulp and 0 to 30 wt % of precipitated calcium carbonate and having a Stockigt sizing degree of not less than 2 seconds and not more than 25 seconds when formed into a sheet having a basis weight of 64 g/m2, and a coating layer composed mainly of white pigment, with the coating layer being formed on at least one side of the substrate at a weight of 1 to 10 g/m2. According to this patent, this sheet has a high ink absorption rate and is able to develop bright colors and sharp images.
U.S. Pat. No. 4,592,954 describes a transparency for ink jet printing comprised of a supporting substrate and thereover a coating consisting essentially of a blend of carboxymethyl cellulose, and polyethylene oxides. Also disclosed are papers for use in ink jet printing comprised of a plain paper substrate and a coating thereover consisting essentially of polyethylene oxides.
U.S. Pat. No. 5,342,688 describes an ink-receptive sheet comprising a transparent substrate bearing on at least a major surface thereof an ink-receptive layer which comprises at least one imaging polymer and an effective amount of polymeric mordant, which comprises a polymethylene backbone carrying pendant aminoguanidino groups.
U.S. Pat. No. 4,547,405 describes an ink jet recording sheet comprising a transparent support carrying a layer comprising a mixture of a coalesced block copolymer latex of poly(vinyl alcohol) with polyvinyl(benzyl ammonium chloride) and a water-soluble polymer selected from the group consisting of poly(vinyl alcohol), poly(vinylpyrrolidone) and copolymers thereof.
U.S. Pat. No. 4,575,465 describes an ink jet recording sheet comprising a transparent support carrying a layer formed from a mixture of vinyl-pyridine/vinylbenzyl quaternary ammonium salt copolymer and a hydrophilic polymer selected from the group consisting of gelatin, poly(vinyl alcohol), and hydroxypropyl cellulose, and mixtures thereof.
Many of the proposed inkjet recording sheets, which employ a hydrophilic outer layer, however, do not provide adequate drying times because the water in the ink is not rapidly absorbed by the sheet medium. This results in smearing of the image and possible blocking problems with other recording sheets.
Thus, there is a need for a film material which has good physical characteristics for a printable substrate and which will provide for very rapid ink drying times.
According to the present invention, a film having a water wettable surface suitable for use in inkjet printing is provided.
More specifically, the present invention relates to providing a biaxially oriented film having a water wettable surface, said film comprising a porous high density polyethylene HDPE surface layer and a coating consisting essentially of a silicone glycol composition impregnated in the pore space of said surface layer.
The porous surface layer can contain an HDPE matrix and a network of interconnecting pores communicating throughout the porous surface layer.
In one embodiment, the porous surface layer will also contain a cavitating agent, such as calcium carbonate. Use of a cavitating agent will result in the formation of a network of interconnecting pores during biaxial orientation.
In one embodiment, a core layer and the porous surface layer are co-extruded. The co-extruded film may be biaxially oriented from about 5 to 8 times in the machine direction and from about 6 to 15 times in the transverse direction.
The porous HDPE surface layer may be impregnated with the silicone glycol coating after the film is biaxially oriented.
The water wettable surface layer is receptive to ink from an inkjet printer.
The present invention provides films having the advantage of good physical characteristics for an ink-jet printing medium and very rapid drying times for the water based inks used in ink jet printing.